properties of ti-6al-7nb titanium alloy nitrocarburized ... · properties of ti-6al-7nb titanium...

Acta of Bioengineering and Biomechanics Original paperVol. 19, No. 4, 2017 DOI: 10.5277/ABB-00892-2017-03

Properties of Ti-6Al-7Nb titanium alloynitrocarburized under glow discharge conditions

ANITA KAJZER1*, OLA GRZESZCZUK1, WOJCIECH KAJZER1, KATARZYNA NOWIŃSKA2,MARCIN KACZMAREK1, MICHAŁ TARNOWSKI3, TADEUSZ WIERZCHOŃ3

1 Faculty of Biomedical Engineering, Silesian University of Technology, Zabrze, Poland.2 Faculty of Mining and Geology, Silesian University of Technology, Gliwice, Poland.

3 Warsaw University of Technology, Faculty of Materials Science and Engineering, Warsaw, Poland.

Purpose: The paper presents the results of physicochemical and mechanical properties of the Ti-6Al-7Nb alloy with surface modi-fied by formation of a diffusive nitrocarburized layer deposited in a low-temperature plasma process. The main aim of the study was toevaluate the influence of steam sterilization and exposure to Ringer’s solution on the utility properties of the alloy. Methods: Based onthe study of the microstructure, roughness, wettability, resistance to pitting corrosion, ion infiltration and mechanical properties, theusefulness of the proposed method of surface treatment for clinical application was proven. Results: Deposition of the nitrocarburizedlayer increased the surface roughness and surface hardness, but also reduced the contact angle, and corrosion resistance with respect tothe polished surfaces. The nitrocarburized layer is a barrier against the infiltration of ions to the solution and sterilization and exposure toRinger solution have greater effect on the physicochemical properties rather than on the mechanical ones. Conclusion: It was found thatsterilization, and exposure to Ringer’s solution greatly affect the change of physicochemical properties rather than mechanical propertiesfor both nitrocarburized layers and the Ti-6Al-7Nb alloy of mechanically polished surface.

Key words: Ti-6Al-7Nb alloy, diffusive nitrocarburized layer, electrochemical properties, mechanical properties

1. Introduction

Ti6Al4V alloy of a two-phase + β structure isused extensively in medicine [3]. Its properties shouldbe adapted to the properties of bone and other biome-chanical conditions [16]. However, on the basis ofclinical observations disadvantageous interaction ofAl and V with a human body was found. Vanadiumcauses cytotoxic reactions and, consequently, neuro-genic disorders while aluminum affects the softeningof bones, damages nerve cells and may cause diseasesof brain and blood vessels [15]. Hence, Semlitschet al. [20] studied in detail Ti6Al(3,5-9,5)Nb(1-6)Tatitanium alloys, in which vanadium was eliminated,and replaced with niobium and tantalum. They gaineda better corrosion resistance and mechanical proper-ties compared to the Ti6Al4V alloy. Similar research

was also conducted by authors of papers [3], [10],[13], [17], [18], who also found a negative influenceof these elements and the need to replace them withelements of greater biocompatibility.

Ti-6Al-7Nb alloy does not contain vanadiumwhich was replaced by niobium. This element and itsoxides, including Nb2O3, belong to the group of com-pounds inert to a body. Furthermore, due to the greaterreactivity, compounds of Nb with O2 are formed moreeasily than compounds of Al with O2. Nb oxides, dur-ing the passivation process, form a compact structureof the surface layer that is more resistant to corrosionin the environment of tissues and body fluids, thusreducing the number of post-implantation complica-tions. This issue is associated with greater resistanceof these alloys to pitting corrosion [15], [24]. Despitethe good corrosion resistance of Ti-6Al-7Nb alloy, itdoes not have good resistance to abrasive wear, which

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* Corresponding author: Anita Kajzer, Silesian University of Technology, Faculty of Biomedical Engineering, ul. Generała Charlesade Gaulle’a 72, 41-800 Zabrze, Poland. Phone: 48(32)277-74-22, E-mail: Anita.Kajzer@polsl.pl

Received: April 20th, 2017Accepted for publication: July 12th, 2017

A. KAJZER et al.182

can be enhanced by plastic treatment, but mostly dueto different methods of surface engineering, such aselectrochemical oxidation, sol-gel method, CVD pro-cesses, gas and glow discharge nitriding or innovativeprocesses and oxynitriding and carbonitration in low-temperature plasma [1], [2], [4], [5], [9], [12], [14],[21], [23], [25], [26]. Therefore, the main aim of thiswork was to compare the mechanical and physico-chemical properties of the Ti-6Al-7Nb alloy with ni-trocarburized layer in the initial state, after steriliza-tion, and exposure to Ringer’s solution, respectively.

2. Materials and methods

Studies were carried out on the Ti-6Al-7Nb alloy(Protasul R100). The samples were in the form ofdiscs 14 mm in diameter and 3 mm thick. This alloywas characterized by properties consistent with therecommendation of ISO 5832-11 standard. In order toobtain a roughness of Ra < 0.1 µm, the surface waspolished using an abrasive paper of 320 and 1200 grit,and then mechanically polished using a silica suspen-sion. Next, the diffusion surface layer of Ti(CN)+ Ti2N + Ti(N) was deposited in the nitrocarburizingprocess in a low-temperature plasma. A hybrid proc-ess combining glow discharge nitriding at 750 °C underthe reactive atmosphere consisting of N2 + 5% H2 at thepressure of 180 Pa for 4 hours with the process ofcarburization in the low-temperature plasma carriedout in the same technological cycle by changing thegaseous atmosphere. In the case of glow discharge nitro-carburization the mixture of N2 + CH4 (10 vol. %) wasused at the pressure of 120 Pa for 1 h. In this way, asdemonstrated by our earlier study [6], in a first step,the TiN + Ti2N + Ti(N) layer is formed, wherein theouter layer zone, titanium nitride (TiN), is non-stoichio-metric as it contains approx. 32% of nitrogen, witha developed surface with defects in the crystal structure,which allows, during changing the reactive atmosphere(second process step), the diffusion of carbon in theamount up to 16–18% (atomic). In this manner, theouter zone of titanium carbonitride – Ti(C, N) wasobtained.

The microstructure of the deposited layers wastested using an light microscopy (Nikon microscopeLV150N). The samples were cut on the IsoMet 1000saw (Beuhler), then included in the resin with (OPAL410, ATA). Grinding was performed using sandpaperof 240–2000 grit with the use of automatic grindingand polishing machine Saphir 530 (ATA). Polishingwas carried out using a suspension of SiO2 with

a grain size of 0.2 μm. Next, the samples were etchedwith the following solution: H2O + 2 ml HNO3 + 2 mlHF. The chemical composition (distribution of Ti, C,N, Al from the surface) of the deposited layer wascarried out by the WDS method using the CAMECSU-30 microprobe.

The layers deposited in the glow discharge nitrocar-burization process as well as the polished Ti-6Al-7Nballoy in the initial state were subjected to steam steriliza-tion using the HMT-260F autoclave (HMC EUROPE),with steam at 135 °C at the pressure of 2 atm as thesterilizing agent, for 60 min. To simulate the tissueenvironment the samples were exposed to the Ringer’ssolution for 28 days at 37 ± 1 °C in the incubator(Binder).

The samples were subsequently divided into sixgroups which were designated according to the fol-lowing scheme: Ti67_p – mechanically polished titanium alloy, Ti67_p_s – mechanically polished titanium alloy

after sterilization, Ti67_p_s_e – mechanically polished titanium al-

loy after sterilization and exposition to Ringer’ssolution,

Ti67(C,N) – mechanically polished titanium alloywith Ti(CN) + Ti2N + Ti(N) layer deposited atthe 750 °C,

Ti67(C,N)_s – mechanically polished titaniumalloy with Ti(CN) + Ti2N + Ti(N) layer depos-ited at the 750 °C after sterilization,

Ti67(C,N)_s_e – mechanically polished titaniumalloy with Ti(CN) + Ti2N + Ti(N) layer depos-ited at the 750 °C after sterilization and expositionto Ringer’s solution.To evaluate the influence of sterilization and expo-

sure to Ringer’s solution on the mechanical and physico-chemical properties of the Ti-6Al-7Nb alloy beforeand after the proposed surface modification, electro-chemical, morphology, surface wettability and me-chanical properties studies have been conducted.

In the first stage of the study the surface roughnesswas measured using the Surtronic S-100 profilometer(Taylor Hobson). Measurements of the Ra and theRz parameters were made for the total measured lengthequal to ln = 4 mm in accordance with the recommenda-tions of the PN-EN ISO 4287: 1999 / A1: 2010.

Surface wettability and surface energy (SEP) wereevaluated using the Owens–Wendt method. The wet-ting angle measurements used two liquids: distilledwater (θw) (by Poch S.A.) and diiodomethane (θd) (byMerck). A measurement with a drop of water anddiiodomethane, placed on the outer layer of the mate-rial, was performed at the temperature T = 23 °C at

Properties of Ti-6Al-7Nb titanium alloy nitrocarburized under glow discharge conditions 183

the test stand incorporating a goniometer SurftensUniversal by OEG and a computer with Surftens 4.5software to analyze the recorded drop image. 5 dropsof distilled water and diiodomethane were appliedonto the surface of each sample, each with capacity of1.5 μl. The measurement began 20 seconds after ap-plication of the drops. Duration of a single measure-ment was 60 seconds with the sampling rate of 1 Hz.Next, the determined values of contact angles andsurface energy γs were presented as mean values withstandard deviation.

Study of pitting corrosion resistance was performedaccording to the PN-EN ISO 10993-15 in Ringer solu-tion of the following composition: NaCl = 8.3 g/dm3,KCl = 0.3 g/dm3, CaCl2 = 0.33 g/dm3 at the tempera-ture of T = 37 ± 1 °C and pH = 7 ± 0.2.

The study was carried out by means of the potenti-odynamic method using three electrodes: the referenceelectrode (saturated calomel electrode – SCE), theauxiliary electrode (platinum electrode), and theworking electrode – the test sample. The study wasrealized with the use of the VoltaLab PGP 201 poten-tiostat. Corrosion tests started from recording the opencircuit potential EOCP. Polarization curves were re-corded from the initial potential Einit = EOCP − 100 mV.The applied scan rate was equal to 1 mV/s. Oncethe anode current density had reached the value of1 mA/cm2, the direction of polarization was changed.On the basis of the obtained curves the following cor-rosion parameters were determined: corrosion poten-tial Ecorr (mV), transpassivation potential Etr (mV)and, using the Stern method, polarization resistanceRp (kcm2).

Measurements of metal ions (Ti, Al, Nb) concen-tration present in the Ringer’s solution resulting fromits effect upon the sample surface during 28 days, weremade with the use of JY2000 spectrometer and atomicemission method with inductively coupled plasma,ICP-AES (Inductively Coupled Plasma – AtomicEmission Spectrometry). The induction source wasa plasma burner linked to 40.68 MHz frequency gen-erator. To draw the reference curve, diluted referencematerials by Merck were used.

The abrasive wear tests using the “3 rollers – cone”method were carried out in accordance with the PN-83/H-04302 standard. As the counter-sample the cone(120°) made of heat-treated (quenching and temper-ing) C45 steel was used. As the test specimens three 8 21 mm rollers were used. The linear velocity incontact with the sample was equal to 0.56 m/s, and theloading was equal to 200 MPa. The depth of wear(linear wear) formed on a cylindrical sample is calcu-lated from the formula (1):

)(5,0 22 aDDh (1)

where: h – the depth of wear (linear wear), D – nomi-nal diameter of the counter-sample, a – the length ofthe major axis of wear trace [mm].

Scratch tests of the nitrocarburized layers on theTi-6Al-7Nb alloy substrate were performed with theuse of the open platform equipped with the Micro--Combi-Tester (CSM) in accordance with the PN-ENISO 20502 standard. The study consisted in makingthe scratch with the use of the penetrator – Rockwelldiamond cone – with a gradual increase of the normalforce. For the evaluation of scratch resistance a recordof the friction force Ft and the penetration depth Pdwere used. The tests were performed with the in-creasing loading force Fc from 0.03 N to 30 N and atthe following parameters: loading rate vs = 10 N/min,the speed of the table vt = 1 mm/min, the length of thescratch l = 3 mm. For each sample three measure-ments were carried out.

In the last stage, hardness and Young’s modulusmeasurements of the deposited layers were con-ducted with the use of the open platform equippedwith a Micro-Combi-Tester (CSM Instruments) usinga Vickers indenter. The Oliver & Phar method was ap-plied. The aim of the study was to determine the instru-mental hardness versus distance from the surface.Loading and unloading rate was equal to 3000 nm/min,a hold time of the sample at maximum load equal to 5 s.The value of the indenter load was resulting. Micro-hardness measurements were carried out for the nitro-carburized samples at the following depths of thepenetrator: 0.5 μm, 1 μm, 2 μm, 4 μm, 6 μm, 8 μm,1 μm, 12 μm, 14 μm and 16 μm. Additionally, fourhardness measurements at depths of 0.5 μm, 1 μm,2 μm, 4 μm for samples without the layer at the load-ing from 25 mN to 1.25 N were carried out.

3. Results

Figure 1 shows the microstructure of the Ti-6Al-7Nballoy after the hybrid glow discharge nitrocarburizingand the distribution of Ti, N, Al and C in the layer.

The nitrocarburized Ti(CN) + Ti2N + Ti(N) layerwas characterized by a uniform thickness of the zonecompounds Ti(CN) + Ti2N of c.a. 15 μm across thecross-section of the sample. The thickness of the dif-fusion zone, as the result of nitrogen diffusion into theTi-6Al-7Nb alloy, was approx. 40 μm.

Based on the analysis of the parameters derivedduring the evaluation of surface roughness it was

A. KAJZER et al.184

found that for samples without the layer the Ra parame-ter, for all analyzed samples, was equal to 0.05 µm. Forthe nitrocarburized layer, the increase in Ra and Rz inrelation to the samples in the initial state was observed(Fig. 2). The results of the contact angle test and sam-ple drops applied on the surface of the samples sub-jected to mechanical polishing and samples with thenitrocarburized layer are shown in Figs. 3 and 4.

Fig. 2. Results of surface roughness measurement, parameters

Fig. 3. The surface energy calculatedbased on the contact angle measurements – OW method

Fig. 4. Measurement of wetting angle, for the group:a) Ti67_p (θav = 59.90°), b) Ti67 (C, N) (θav = 92.16°)

It was found that for all the samples before theglow discharge nitrocarburizing (sample: Ti67_p,Ti67_p_s and Ti67_p_s_e) no difference in the val-ues of contact angle and surface energy was ob-served (ie., Ti67_p_s_e). But for the samples with theTi(C, N) + Ti2N + Ti(N) layer (sample: Ti67 (C, N)and Ti67 (C, N)_s_e of larger surface roughness) de-creased wettability of the surface was observed. Re-spectively, the average value of the contact angle wasequal to θav was 92.16° and 98.83°.

Based on the recorded corrosion parameters, forall the analyzed groups (Fig. 5) a decrease of corro-sion resistance of the alloy Ti-6Al-7Nb with the nitro-carburized layer was observed (Table 1), what wasmainly caused by development of the surface after thenitrocarburizing (Fig. 5).

The Ti-6Al-7Nb alloy with the polished surface isresistant to pitting corrosion in the entire measuringrange. On the other hand, sterilization (Ti67 (C,N)_s)and exposure to Ringer’s solution (Ti67_p_s_e, Ti67(C, N)_s_e) has increased the value of the corrosion

(a) (b)

Fig. 1. Microstructure (a) and distribution of elements from the face of the sample (b)after the glow discharge nitrocarburizing in the low-temperature plasma

Properties of Ti-6Al-7Nb titanium alloy nitrocarburized under glow discharge conditions 185

potential Ecorr while reducing the polarization resis-tance Rp. Deposition of the nitrocarburized layercaused an increase of the corrosion potential butconcurrent reduction of the polarization resistanceRp. In this case an occurrence of transpassivationpotential was observed.

Table 1. Results of potentiodynamic tests – mean values

Ecorr, mV Rp, kΩcm2 Etr, mVNo.of group Ti-6Al-7Nb mechanically polishedTi67_p –213 ± 7 1230 ± 155 –

Ti67_p_s –83 ± 32 293 ± 51 –Ti67_p_s_e –154 ± 52 222 ± 55 –

Ti-6Al-7Nb with the layerTi67(C,N) +53 ± 5 13 ± 2 +1061 ± 6

Ti67(C,N)_s +155 ± 11 15 ± 5 +1183 ± 3Ti67(C,N)_s_e +196 ± 13 22 ± 1 +1124 ± 2

Table 2. Results of metal ions infiltration

Metal ions infiltration tests, 2cm

μg (mean value)

Group Ti Al NbTi67_p 97.8 ± 0.01 85.8 ± 0.02 65.9 ± 0.01

Ti67_p_s 88.5 ± 0.01 72.3 ± 0.01 64.6 ± 0.02Ti67(C, N) 65.1 ± 0.02 64.5 ± 0.01 58.6 ± 0.01

Ti67(C, N)_s 57.9 ± 0.01 56.8 ± 0.03 58.4 ± 0.05

Despite the reduced corrosion resistance in relationto the polished alloy, the deposited nitrocarburized layerbeneficially reduces the concentration of ions of metalelements infiltrating to Ringer’s solution (Table 2). Ionsinfiltration studies allowed to determine the presence ofions of the main alloying elements – Ti, Al, Nb. Thegreatest concentration of elements which penetrated tothe solution was observed for Ti ions (97.8 µg/cm2) for

the samples from the Ti67_p group, while the smallesTi (57.9 µg/cm2) was recorded for the samples from theTi67 (C, N)_s group, i.e., the surface coated with thenitrocarburized layer subjected to sterilization process.The sterilization combined with the exposure to Ringer’ssolution for 28 days caused the reduction of concentra-tion of metal ions infiltrating to the solution as com-pared to other variants.

Fig. 6. Resistance to abrasive wear of the Ti-6Al-7Nb alloyin the initial state and after the glow discharged nitrocarburizing

The results of resistance to abrasive wear using the“3 rollers + cone” method are shown in Fig. 6. Theprocess of nitrocarburizing significantly improvedresistance to abrasive wear of the Ti-6Al-7Nb alloy.The Ti-6Al-7Nb alloy in the initial state has beendamaged after the first stage of the test (10 min),whereas in the case of the alloy with the depositedTi(CN) + Ti2N + Ti(N) layer formed Ti (CN) +Ti2N + Ti (N), the linear wear was only approx. 1.2 μmafter 100 minutes.

The results of the scratch resistance of the nitro-carburized layer, including the sterilization, and theexposure to Ringer’s solution, are presented in Fig. 7.The highest scratch resistance was recorded for thenitrocarburized layer after the exposure to Ringer’s

(a) (b)

Fig. 5. Examples of polarization curves for: (a) Ti67_p, Ti67_p_s, Ti67_p_s_e, (b) Ti67(C, N), Ti67(C, N)_s, Ti67(C, N)_s_e

A. KAJZER et al.186

solution (Ti67 (C, N) _p_s_e) for which, the value offriction force was the lowest Ft – Fig. 7. While thesterilization process ((Ti67 (C,N)_s) caused the de-crease in the scratch resistance, compared to the initialstate (Ti67 (C, N)). For this case, the friction force Ftreached the highest value – Fig. 7.

Fig. 7. Example relation of the friction forceas a function of the scratch length

The results of micro-hardness also confirmed thebeneficial influence of the deposited layer on the me-chanical properties of the surface. The results for allthe analyzed samples are shown in Tables 3 and 4 andin Fig. 8.

Based on the performed measurements a slightdifference in the values of hardness and Young’smodulus, depending on the method of surface prepa-ration, was observed. It was found that sterilization

(Ti67_p_s) and exposure to Ringer’s solution(Ti67_p_s_e) resulted in a slight decrease in hard-ness compared to the initial state (Ti67_p). Deposi-tion of the nitrocarburized layer increased the hard-ness, compared to the Ti-6Al-7Nb substrate (Table 4).The highest hardness was measured for the samplesexposed to Ringer’s solution (Ti67 (C, N)_p_s_e)(Table 4, Fig. 8). On the other hand, the sterilizationprocess (Ti67 (C, N)_s) caused a slight decrease inhardness of the nitrocarburized layer compared to theinitial state (Ti67 (C, N)).

Table 3. Hardness of Ti-6Al-7Nb surface

Number of measurment

1 2 3 4

0.5 μm 1 μm 2 μm 4 μm

Ti67_pNanohardnessHIT, MPa 4178 ± 169 4505 ± 142 4296 ± 196 3973 ± 158

Young’s modulusE, GPa 124 ± 18 136 ± 17 143 ± 20 127 ± 19

Ti67_p_sNanohardnessHIT, MPa 3579 ± 132 4094 ± 187 3954 ± 201 3921 ± 140

Young’s modulusE, GPa 127 ± 20 137 ± 15 140 ± 21 130 ± 18

Ti67_p_s_eNanohardnessHIT, MPa 3042 ± 155 4046 ± 173 4026 ± 210 4275 ± 191

Young’s modulusE, GPa 128 ± 16 142 ± 17 149 ± 16 133 ± 14

Table 4. Hardness and Young’s modulus of the nitrocarburized layers on Ti-6Al-7Nb

Number of measurement

1 2 3 4 5 6 7 8 9 10

0.5 μm 1 μm 2 μm 4 μm 6 μm 8 μm 10 μm 12 μm 14 μm 16 μm

Ti67(C, N)

Nanohardness HIT, MPa 6549± 214

6687± 193

6448± 199

5384± 176

5015± 185

4700± 169

4525± 154

4420± 149

4200± 183

4207± 205

Young’s modulus E, GPa 152± 16

160± 18

135± 15

130± 20

121± 21

117± 19

114± 22

112± 17

110±18

107± 16

Ti67(C, N)_s

Nanohardness HIT, MPa 6466± 208

6339± 191

6259± 178

5375± 206

4650± 170

4626± 185

4619± 172

4490± 188

4227± 176

4271± 206

Young’s Modulus E, GPa 155± 17

139± 16

141± 19

125± 21

115± 18

118± 20

113± 17

111± 18

104± 21

103± 19

Ti67(C, N)_p_s_e

Nanohardness HIT, MPa 6272± 169

6511± 175

6494± 172

6004± 198

5218± 206

5231± 192

4802± 196

4727± 173

4749± 181

4518± 176

Young’s Modulus E, GPa 138± 19

146± 22

150± 20

135± 17

121± 18

118± 20

113± 19

108± 21

106± 17

100± 18

Properties of Ti-6Al-7Nb titanium alloy nitrocarburized under glow discharge conditions 187

Fig. 8. Hardness as a function of the penetration depth

4. Discussion

Based on the results obtained it may be stated thatthe sterilization and exposure to Ringer solution havea greater effect on the physicochemical propertiesrather than on the mechanical ones.

It has been observed that surface development hasa significant influence on wettability, i.e., for thelarger values of the Ra parameter the wettability of thesurface is smaller. The process of the glow dischargenitrocarburizing influenced the hydrophobicity of thesurface. In contrast, the exposure to Ringer’s solutioncaused that the wettability was similar to the polishedsurfaces.

Whereas decrease of corrosion resistance of thealloy Ti-6Al-7Nb with the nitrocarburized layer wasmainly caused by development of the surface after thenitrocarburizing (Fig. 2), it is also connected with thechange in surface energy (Fig. 3), especially after thesterilization, during which on the surface of theTi(C, N) + Ti2N + Ti(N) layer, a nanometer-thick layerof titanium oxides may be formed [6], as observed dur-ing the sterilization of the polished Ti-6Al-7Nb alloy[3], [22]. Deposition of the nitrocarburized layercaused the favorable increase of the corrosion poten-tial but concurrent reduction of the polarization resis-tance Rp. For these surfaces the occurrence of trans-passivation potential was also observed, indicating thereduction of corrosion resistance in relation to thepolished Ti-6Al-7Nb alloy. Simultaneously, for thesterilized and exposed to Ringer’s solution samples,a slight increase in the Etr was recorded. The influenceof the deposited layer on electrochemical propertieswas also observed by authors of [11], who found a posi-tive increase of the corrosion potential Ecorr for theTi-6Al-7Nb alloy with the diffusion nitrided layercompared to the biomaterial substrate.

On the other hand, assuming the amount of degra-dation products infiltrating into the body as an impor-tant criterion for demonstrating the biocompatibilityof the studied material, it must be concluded that thedeposited nitrocarburized layer affects the barrierproperties. The greatest concentration of elementswhich penetrated to the solution was observed for Ti.This is, most likely, caused by the presence of stoichi-ometric interlayer of titanium nitride Ti2N underneaththe carbonitride Ti(C, N) layer [9].

The obtained results of the scratch resistance ofthe nitrocarburized layer, including the sterilization,and the exposure to Ringer’s solution indicated slightdifferences in the friction force Ft depending on thesurface preparation. Similar research conducted byFarokhzadeh K. et al. [8] who produced nitrided layeron the Ti-6Al-4V alloy. They also found the reductionof friction force when using the nitrided samples,compared to the samples in the initial state. The val-ues of hardness obtained in the present work arehigher in comparison to studies conducted by Fare [7].In turn, the nitrided layer obtained by Rahman [19] onthe Ti-6Al-4V substrate was characterized by signifi-cantly higher values of hardness.

5. Conclusions

Based on the obtained results the following can bestated:– deposition of the nitrocarburized layer increased

the surface roughness and surface hardness butalso reduced the contact angle, and corrosion re-sistance with respect to the polished surfaces,

– the nitrocarburized layer is a barrier against theinfiltration of ions to the solution, as a consequenceof the presence of the transition zone – stoichi-ometric Ti2N between the outer layer of titaniumcarbonitride (Ti (CN)) with a developed topographyand the Ti(N) diffusion zone [6], [27],

– deposition of the Ti(CN) + Ti2N + Ti(N) layer onthe Ti-6Al-7Nb alloy significantly increases its re-sistance to abrasive wear,

– sterilization and exposure to Ringer solution havea greater effect on the physicochemical propertiesrather than on the mechanical ones.Summarizing, instead of increase in pitting corro-

sion resistance, it can be concluded that the proposedmethod of surface modification contributed to theimprovement of utility properties. This involves thepossibility of a wider use of Ti-6Al-7Nb alloy forimplants in bone surgery.

A. KAJZER et al.188

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